1. Field of the Invention
The present invention relates to an arc length control technique adapted for pulse arc welding of consumed electrode type using, as shield gas, carbon dioxide (gas) or a gas mixture containing carbon dioxide as a main component.
2. Description of the Related Art
From the necessity of properly keeping an arc length to maintain satisfactory welding quality, there is known an arc length control technique for suppressing changes of the arc length during the welding (see Japanese Unexamined Patent Application Publication No. 2002-361417 and Japanese Patent No. 3147046). Qualitatively, the arc length is determined based on balance between a feed speed of a welding wire (i.e., a wire feed speed) and a melting rate of the wire. More specifically, if the wire feed speed varies during the welding due to, e.g., feed resistance caused in a wire feed path, the balance between the wire feed speed and the melting rate is lost, thus causing a variation in the arc length. Also, the arc length varies due to, e.g., changes of the tip-to-base material distance (i.e., the distance between a tip and a base material), which are caused by, e.g., shaking of hands of a welding worker. In order to suppress the variation in the arc length caused by the above-mentioned disturbances, therefore, a welding current corresponding to the melting rate needs to be adjusted depending on changes of a welding voltage corresponding to the arc length. Arc length control described in Japanese Unexamined Patent Application Publication No. 2002-361417 will be described below.
In a general pulse arc welding of consumed electrode type, a gas mixture of Ar-5 to 30% of CO2 is used as shield gas (MAG pulse welding). In the known technique, as illustrated in FIG. 15, waveforms of a welding current and a welding voltage are controlled respectively by using a pulse current waveform and a pulse voltage waveform (referred to collectively as a “pulse waveform”). In FIG. 15, the vertical axis of the waveform illustrated in the upper side represents a detected value Io of the welding current, and the vertical axis of the waveform illustrated in the lower side represents an instantaneous value Vo of the welding voltage. The horizontal axis of each of the waveforms represents time t.
In the welding control illustrated in FIG. 15, a higher welding current than a mean welding current is supplied during a peak period Tp to release (separate) a droplet of molten metal. Correspondingly, a welding voltage value during the peak period Tp becomes Vp. During a base period Tb subsequent to the peak period Tp, a lower welding current than the mean welding current is supplied to avoid transfer of the droplet. Correspondingly, a welding voltage value during the base period Tb becomes Vb. By repeating the peak period Tp and the base period Tb as a combined pulse cycle Tpb (one cycle), the transfer of one droplet per pulse is carried out in sync with the pulse waveform. As a result, welding can be performed with less sputtering than the case not using the above-described pulse waveform.
In the upper side of FIG. 15, a time mean value of a waveform area indicated by hatching in a period between a start time t(n) of the n-th pulse and a start time t(n+1) of the (n+1)-th pulse, i.e., in the n-th pulse cycle Tpb(n), represents a mean welding current value Iw(n). Similarly, in the lower side of FIG. 15, a time mean value of a waveform area indicated by hatching represents a mean welding voltage value Vw(n).
In general, because stability of an arc length control system is greatly affected by the gradient of an external characteristic of a welding power supply, the gradient of the external characteristic needs to be set to a proper value depending on, e.g., welding conditions (i.e., setting of the wire feed speed, the welding voltage, etc.), the wire type, and the composition of the shield gas. FIG. 16 illustrates one example of the gradient Ks of the external characteristic set depending on a certain case of the welding conditions, the wire type, and the composition of the shield gas. The arc length control can be realized by performing output control such that, in the certain case of the welding conditions, the wire type, and the composition of the shield gas, an operating point for the welding current and the welding voltage during the welding is kept positioned on a straight line in FIG. 16, which represents the gradient Ks of the external characteristic. This has the same meaning as performing output control in such a way that the mean welding current value Iw(n) and the mean welding voltage value Vw(n), illustrated in FIGS. 15 and 16, satisfy the relationship expressed by the following formula (101). In the formula (101), Is is a setting value of the welding current, which indicates a setting condition set in advance, and Vs is similarly a setting value of the welding voltage.Vw(n)=Ks{Is−Iw(n)}+Vs  (101)
In FIG. 16, for example, a position P1 of the setting condition is set as a reference position. When the arc length increases during the welding, the detected value of the welding voltage increases and the mean welding voltage value Vw(n) becomes higher than the setting value Vs of the welding voltage, as seen from FIG. 16. In that case, because the operating point is moved from the position P1 to P2 on the straight line representing the gradient Ks of the external characteristic, the mean welding current value Iw(n) becomes lower than the setting value Is of the welding current, as seen from FIG. 16. Hence the wire melting rate reduces, whereby the arc length also reduces. Consequently, the operating point is moved in a direction converging to the position P1.
Conversely, when the arc length reduces during the welding, the detected value of the welding voltage reduces and the mean welding voltage value Vw(n) becomes lower than the setting value Vs of the welding voltage, as seen from FIG. 16. In that case, because the operating point is moved from the position P1 to P3 on the straight line representing the gradient Ks of the external characteristic, the mean welding current value Iw(n) becomes higher than the setting value Is of the welding current, as seen from FIG. 16. Hence the wire melting rate increases, whereby the arc length also increases. Consequently, the operating point is moved in a direction converging to the position P1.
Thus, setting the external characteristic having the proper gradient Ks is equivalent to controlling a change amount of the welding current depending on a change of the welding voltage. As a result, the change amount of the arc length can be suppressed. In more detail, a welding power supply apparatus described in Japanese Unexamined Patent Application Publication No. 2002-361417 performs the arc length control as follows. Here, assuming the instantaneous value of the welding current and the instantaneous value of the welding voltage detected at a certain point in time to be Io and Vo, respectively, a voltage error integral value Svb corresponding to errors between respective detected values and the setting value Vs of the welding voltage and the setting value Is of the welding current within a pulse cycle is defined by the following formula (102):Svb=∫{Ks(Io−Is)+Vs−Vo}dt  (102)
In the welding power supply apparatus described in Japanese Unexamined Patent Application Publication No. 2002-361417, calculation of Svb in the formula (102) is started at the time t(n) when the n-th pulse cycle Tpb(n) has started. Then, at a time when Svb=0 is resulted during the n-th base period Tb after the end of the preset n-th peak period Tp, the n-th pulse cycle Tpb(n) is brought to an end. By setting that time to be t(n+1), the (n+1)-th pulse cycle is started. As a result of repeating the above-described steps, the operating point can be held on a line corresponding to the gradient Ks of the external characteristic represented by the above formula (101), and the arc length control can be realized in such a way that the arc length is adjusted in units of one pulse cycle (one droplet).
On the other hand, the technique described in Japanese Patent No. 3147046 relates to arc welding of consumed electrode type using shield gas containing carbon dioxide as a main component, and it performs arc length control by generating a complex pulse waveform as illustrated in FIG. 17. A pulse waveform generation means described in the above-cited Japanese Patent employs constant current control and constant voltage control in a combined manner. The pulse waveform illustrated in FIG. 17 alternately includes a predetermined peak period Tp and a predetermined base period Tb. The pulse waveform generation means outputs a constant voltage corresponding to an initial voltage Vc during an initial peak period Tc that starts from the same start point as the peak period Tp. In a subsequent peak period (Tp−Tc), the pulse waveform generation means outputs a constant peak voltage Vp. Further, the pulse waveform generation means performs, during the base period Tb, the constant current control based on a pulse waveform provided by a predetermined base current value Ib.
Moreover, with the technique described in Japanese Patent No. 3147046, after a droplet release detection means has detected release of a droplet of molten metal, the pulse waveform generation means outputs a predetermined current value Ir only during an output correction time Tr. In the peak period immediately after the elapse of the output correction time Tr, the pulse waveform generation means performs, without generating the waveform in the initial peak period, the constant voltage control such that the detected voltage is held at the peak voltage Vp over an entire region of the pulse waveform. The initial peak period Tc and the initial voltage Vc are set to prevent the pulse current from rising excessively. In other words, the initial peak period Tc and the initial voltage Vc are set to proper values to minimize an arc force that is imposed on the droplet when the droplet is released in an initial stage of the peak period.
In addition, with the technique described in Japanese Patent No. 3147046, the base period Tb is set as a fixed parameter. Therefore, when the balance between the wire feed speed and the melting rate is lost due to disturbances, the arc length control can be performed so as to compensate for the arc length by increasing or reducing the pulse peak current during the peak period (Tp−Tc), or by increasing or reducing the pulse peak current during the peak period Tp immediately after the output correction time Tr. For that reason, the technique described in Japanese Patent No. 3147046 is suitable for such a situation that the tip-to-base material distance and the welding conditions are hardly changed during the welding.
The inventors of this application have previously proposed a pulse arc welding method using, as shield gas, carbon dioxide alone or a gas mixture containing carbon dioxide as a main component and alternately outputting two types of pulse waveforms having different pulse peak current levels per cycle, wherein an arc length is controlled to be constant by adjusting one or more of a peak current, a base current, a peak period, and a base period of a second pulse, which serves to shape a droplet of molten metal, within such an extent that one droplet is transferred per cycle and that the transfer of one droplet per cycle is not disturbed even when the distance between a contact tip and a base material distance is changed (see Japanese Unexamined Patent Application Publication No. 2007-237270).
However, the related art disclosed in Japanese Unexamined Patent Application Publication No. 2007-237270 still has a room for improvement in the technique of suppressing a variation in the arc length caused by disturbances. From that point of view, the arc length control described in Japanese Unexamined Patent Application Publication No. 2002-361417 is discussed below. The described arc length control is based on precondition that the mean welding voltage value Vw(n) in the n-th pulse cycle Tpb(n) is substantially proportional to the arc length in the n-th pulse cycle. Such precondition is satisfied by the pulse arc welding method in which a gas mixture of Ar-5 to 30% of CO2 is used as the shield gas and the simple pulse waveform, illustrated in FIG. 15, is repeated. However, the above-described precondition cannot be applied to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2007-237270. In other words, the above-described precondition is not satisfied in the pulse arc welding of consumed electrode type in which carbon dioxide alone or the gas mixture containing carbon dioxide as a main component is used as the shield gas and one droplet is transferred per cycle by using two types of pulse waveforms having different peak current levels in one pulse cycle.
Further, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 2007-237270, it is also supposed, for example, to perform welding in a manner of weaving the tip within a gap (groove) such that the tip-to-base material distance is momentarily changed. On the other hand, in the arc length control described in Japanese Patent No. 3147046, since the base period Tb is set as a fixed parameter, the arc length is compensated for by increasing or reducing the pulse peak current during a predetermined period. Therefore, when the arc length control described in Japanese Patent No. 3147046 is applied to the welding that the tip-to-base material distance is momentarily changed with the weaving of the tip within the gap, the peak current is abruptly increased or reduced. For that reason, when the welding is performed in such a manner, the molten metal remaining on a wire after the release of the droplet or the droplet under formation is frequently caused to scatter, as spatters, due to an arc reaction force.
In the technique described in Japanese Patent No. 3147046, because of generating the pulse waveform illustrated in FIG. 17, the current value greatly varies per cycle in the period where the droplet is formed, i.e., the peak period (Tp−Tc) or the peak period Tp immediately after the output correction time Tr. This results in the problem of causing a variation in droplet size and impairing regularity of the droplet transfer.
Moreover, in the pulse arc welding as represented by the technique disclosed in Japanese Unexamined Patent Application Publication No. 2007-237270, a suspensory shaping process for the droplet in the base period Tb is very important. If the base period Tb is too short, the droplet cannot be released by the next pulse. Also, if the base period Tb is too long, the droplet contacts a molten metal pool, thus generating spatters by a short-circuiting. Hence, coping with those problems has been demanded.